Auto merge of #5767 - glennw:image-cache, r=larsbergstrom,jdm

* Simpler image cache API for clients to use.
 * Significantly fewer threads.
   * One thread for image cache task (multiplexes commands, decoder threads and async resource requests).
   * 4 threads for decoder worker tasks.
 * Removed ReflowEvent hacks in script and layout tasks.
   * Image elements pass a Trusted<T> to image cache, which is used to dirty nodes via script task. Previous use of Untrusted addresses was unsafe.
   * Image requests such as background-image on layout / paint threads trigger repaint only rather than full reflow.
 * Add reflow batching for when multiple images load quickly.
   * Reduces the number of paints loading wikipedia from ~95 to ~35.
 * Reasonably simple to add proper prefetch support in a follow up PR.
 * Async loaded images always construct Image fragments now, instead of generic.
   * Image fragments support the image not being present.
 * Simpler implementation of synchronous image loading for reftests.
 * Removed image holder.
 * image.onload support.
 * image NaturalWidth and NaturalHeight support.
 * Updated WPT expectations.

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components/layout/wrapper.rs

@@ -116,7 +116,7 @@ pub trait TLayoutNode {
     fn image_url(&self) -> Option<Url> {
         unsafe {
             match HTMLImageElementCast::to_layout_js(self.get_jsmanaged()) {
-                Some(elem) => elem.image().as_ref().map(|url| (*url).clone()),
+                Some(elem) => elem.image_url().as_ref().map(|url| (*url).clone()),
                 None => panic!("not an image!")
             }
         }